Jeff Masters: What We Learned from Haiyan (so far)

November 14, 2013

A remarkable warming of the sub-surface Pacific waters east of the Philippines in recent decades, due to a shift in atmospheric circulation patterns and ocean currents that began in the early 1990s, could be responsible for the rapid intensification of Super Typhoon Haiyan. Hurricanes are heat engines, which means they take heat energy out of the ocean, and convert it to kinetic energy in the form of wind. It’s well-known that tropical cyclones need surface water temperatures of at least 26.5°C (80°F) to maintain themselves, and that the warmer the water, and the deeper the warm water is, the stronger the storm can get. Deep warm water is important, since as a tropical cyclone tracks over the ocean, it stirs up cooler water from the depths, potentially reducing the intensity of the storm. When both Hurricane Katrina and Hurricane Rita exploded into Category 5 hurricanes as they crossed over a warm eddy in the Gulf of Mexico with a lot of deep, warm water, the concept of the total heat energy available to fuel a hurricane–the Tropical Cyclone Heat Potential (TCHP)–became one that gained wide recognition.

Figure 1. Departure of temperature from average at a depth of 100 meters in the West Pacific Ocean during October 2013, compared to a 1986 – 2008 average. The track and intensity of Super Typhoon Haiyan are overlaid. Haiyan passed directly over large areas of sub-surface water that were 4 – 5°C above average in temperature, which likely contributed to the storm’s explosive deepening. Image credit: Japan Meteorological Agency.

The Pacific Ocean east of the Philippines has the largest area of deep, warm water of anywhere on Earth, and these waters have historically fueled the highest incidence of Category 5 storms of anywhere on the planet. Super Typhoon Haiyan tracked over surface waters that were of near-average warmth, 29.5 – 30.5°C (85 – 87°F.) However, the waters at a depth of 100 meters (328 feet) beneath Haiyan during its rapid intensification phase were a huge 4 – 5°C (7 – 9°F) above average, judging by an analysis of October average ocean temperatures from the Japan Meteorological Agency (Figure 1.) As the typhoon stirred this unusually warm water to the surface, the storm was able to feed off the heat, allowing Haiyan to intensify into one of the strongest tropical cyclones ever observed.

Why was there such unusually warm sub-surface water?
The sub-surface waters east of the Philippines have warmed dramatically over the past twenty years. According to Punet al. (2013), “Recent increase in high tropical cyclone heat potential area in the Western North Pacific Ocean”, the depth to where ocean temperatures of at least 26°C (79°F) penetrates has increased by 17% since the early 1990s, and the Tropical Cyclone Heat Potential has increased by 13%. The warm-up is due to an increase in the surface winds blowing across the region–the trade winds–which have caused a southward migration and strengthening of the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC). The strong trade winds have pushed a large amount of water up against the east coast of the Philippines in the past twenty years, resulting in a rate of sea level rise of 10 mm per year–more than triple the global average of 3.1 mm/yr (Figure 2.) This extra sea level rise contributed to the storm surge damage from Super Typhoon Haiyan. Sea level rise data from Legaspi in the Eastern Philippines shows a rise of about 305 mm (12 inches) since 1949. For comparison, global average sea level rose 7.5″ (190 mm) since 1901. Part of the rise along the eastern Philippine coast is from tectonic processes–the subsidence of the Philippine plate under the Eurasian plate–but most of it is due to the stronger trade winds piling up warm water along the coast, and the fact that warmer waters expand, raising sea level.

Why have the trade winds sped up?
The surface trade winds in the equatorial Pacific are part of the Walker Circulation–a pattern of rising and sinking air along the Equator that the El Nino/La Nina cycle influences. A strong Walker circulation means there is lower pressure over Indonesia, which pulls in more air at the surface along the Equator from the east, increasing the easterly trade winds. As these trade winds strengthen, they pull surface ocean waters away from South America, allowing cold water to upwell to the surface. This is a La Niña-like situation, which takes heat energy out of the atmosphere, putting it into the ocean, keeping global surface temperatures cooler than they would otherwise be. A weakened Walker circulation is the reverse, resulting in weaker trade winds, and a more El Niño-like situation with higher global surface temperatures. As long as the stronger Walker circulation that has been in place since the early 1990s holds, global surface temperatures should stay cooler than they otherwise would be, prolonging the slow-down in global surface warming that has received much attention this year. There may also be a greater chance of super typhoons and higher storm surges affecting the Philippines, due to the warmer sub-surface waters and re-arranged ocean currents. A 2013 paper by L’Heureux et al.notes that the climate models predict that the Walker circulation should weaken (a more El Niño-like situation)–the reverse of what has been observed the past twenty years. The researchers took the observed pressure patterns over the Pacific in recent decades and removed the atmospheric response to the El Niño/La Niña cycle. The resulting pattern they found showed a steady strengthening of the Walker circulation, in concert with global rising temperatures. So, are we seeing a failure of the climate models? Or is the recent speed-up of the Walker circulation a decades-long temporary “speed bump” in the climate system? Time will tell. It is worth pointing out that a just-released paper by British and Canadian researchers shows that the global surface temperature rise of the past 15 years has been greatly underestimated. As discussed at realclimate.org, “The reason is the data gaps in the weather station network, especially in the Arctic. If you fill these data gaps using satellite measurements, the warming trend is more than doubled in the widely-used HadCRUT4 data, and the much-discussed “warming pause” has virtually disappeared.”
(note: see the Realclimate posted on this blog above)

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2 Responses to “Jeff Masters: What We Learned from Haiyan (so far)”

Am I the only one that received this post? You would think that at the very least “Sea Level Dave” would have commented reflexively, since it mentions that small peg he hangs his hat on as he ignores all other evidence. Perhaps he really read and understood the piece and decided to stay away from it? Smart move on his part.

Excellent graphics and commentary from Jeff, even though he used a few “coulds” and “likelies”. The thrust of the piece leaves little doubt in my mind that we have yet another combination of factors due to AGW that is going to be particularly hard on the Philippines in the future.

(And it drives yet another nail in the coffin lid. Is there NO good news out there?)

Generally since the start of thorough research (c. 1950) until today, Haiyan (895 hPa and 315 km / h) is the fourth most intense tropical cyclone ever observed. Ahead of it: Tip (870 hPa, 315 km / h) from 1979 (“took” half of the United States!), Camille in 1969 and Nancy in 1961 (12 September has reached today, unbeaten record speed: 345 km / h). Haiyan had a relatively high pressure in its center. In this regard Haiyan not is located even in the first 12 strongest typhoons (875 – 885 mbar – http://en.wikipedia.org/wiki/Typhoon_Tip)! What is interesting, the first 11 in this classification of the most intense typhoons, has happened between 1953 and 1984. It (mainly) was a cool phase of the PDO (http://www.climatedata.info/resources/Forcing/Oscillations/04-Pacific-Decadal-Oscillation-index.gif). Between 1984 and 2010, in the positive phase of the PDO, was not any typhoon from the list of 12 most intense – if we take into account the measured intensity of the low pressure at its center. 9-12 ex aequo “took” place, here, Megi – 2010. And about 2008 years slowly began to start again cool PDO phase …